The present application relates to a positive electrode active material, a positive electrode using the same and a non-aqueous electrolyte secondary battery. For example, the present application relates to a positive electrode active material for non-aqueous electrolyte secondary battery containing a complex oxide containing lithium (Li) and a transition metal, a positive electrode using the same and a non-aqueous electrolyte secondary battery.
In recent years, techniques of portable electronic appliances have conspicuously developed, and electronic appliances such as mobile phones and laptop personal computers have started to be recognized as a fundamental technology for supporting the highly computerized society. Also, research and development regarding high functionalization of these electronic appliances are energetically advanced, and power consumption of these electronic appliances increases steadily in proportion thereto. On the other hand, these electronic appliances are required to be driven for a long period of time, and densification of high energy of a secondary battery which is a drive power source has been inevitably desired. Also, the prolongation of a cycle life has also been desired in view of environmental consideration.
From the viewpoints of occupied volume and mass of a battery to be built in an electronic appliance, it is desired that the energy density of the battery is as high as possible. At present, a lithium ion secondary battery is built in almost all of appliances because it has a high voltage and an excellent energy density as compared with other battery systems.
In general, the lithium ion secondary battery uses lithium cobaltate for a positive electrode and a carbon material for a negative electrode, respectively and is used at an operating voltage in the range of from 4.2 V to 2.5 V. In a unit cell, the fact that a terminal voltage can be increased to 4.2 V largely relies upon excellent electrochemical stability of a non-aqueous electrolyte material, a separator and so on.
For the purposes of realizing higher performances and enlarging applications of such a lithium ion secondary battery, a number of investigations have been advanced. As one of them, for example, it is investigated to increase the energy density of a positive electrode active material starting with lithium cobaltate by a method for increasing a charging voltage or the like, thereby contriving to attain a high capacity of the lithium ion secondary battery.
However, in the case of repeating charge and discharge at a high capacity, there is involved a problem that deterioration of the capacity is caused, whereby the battery life becomes short. Also, when used in a high-temperature environment, there is involved a problem that a gas is generated in the inside of the battery, resulting in causing liquid leakage, deformation of the battery, etc. Then, there has hitherto been employed a method for modifying a positive electrode active material by mixing a small amount of LiMn1/3Co1/3Ni1/3O2, etc. in the positive electrode active material and using the mixture or coating the surface of the positive electrode active material with other material.
For example, Japanese Patent No. 3172388 discloses a method for improving a cycle characteristic by coating a metal oxide on the surface of a positive electrode. Also, Japanese Patent No. 3691279 discloses a method for enhancing thermal stability by coating a metal oxide on the surface of a positive electrode active material.
Also, in coating the surface of a positive electrode active material, effects for improving a cycle characteristic and enhancing thermal stability by a coating form thereof are also investigated. For example, JP-A-7-235292, JP-A-2000-149950, JP-A-2000-156227, JP-A-2000-164214, JP-A-2000-195517 and JP-A-2002-231227 disclose a method for uniformly coating a lithium transition metal complex oxide. Also, JP-A-2001-256979 discloses a positive electrode active material having a lump of a metal oxide deposited on a metal oxide layer.
Also, elements to be used for surface coating are investigated. For example, JP-A-2002-164053 discloses a positive electrode active material having one or more surface treatment layers containing two or more coating elements on the surface of a lithium compound as a core.
JP-A-2003-7299 discloses a method for improving thermal stability by disposing a surface treatment layer containing a compound represented by MXOk on the surface of a particle. Also, JP-A-2006-127932 discloses a method for forming a surface layer containing a compound represented by MlPmOn.
JP-A-05-36411 discloses a method for using a positive electrode having phosphorus (P) added thereto; and Japanese Patent No. 3054829 discloses a material in which the surface of a particle is coated with phosphorus (P). Also, Japanese Patent No. 3192855 discloses a method for forming a layer containing boron (B), phosphorus (P) or nitrogen (N).
Also, JP-A-10-154532, JP-A-10-241681 and JP-A-11-204145 disclose a method for containing a phosphoric acid salt compound, etc. in a positive electrode.